The cyclins are a class of polypeptides which are involved in the control of the cell cycle. Three closely related human D-type cyclins have been identified, all of which interact with and activate cyclin dependent kinases (CDK) 4 and 6, although they have specialized function in distinct cell types. The cyclin D1 gene has been found to be overexpressed and/or deregulated by clonal chromosome rearrangements or by amplification in B cell lymphoma, in parathyroid adenoma, and in breast and squamous cell cancer. It has also recently been shown that cyclin D1 deficient mice have a defect in estrogen-mediated proliferation of breast epithelium during pregnancy (Sicinski et al, 1995, Cell 82; 621-630; Fantl et al, 1995, Genes & Development 9; 2364-2372).